Conventionally, the development of direct conversion type compound semiconductors having high efficiency and high resolution and being excellent in miniaturization is proceeding in radiation detection element applications. Among them, cadmium telluride (CdTe), cadmium selenium telluride (CdSeTe), cadmium zinc telluride (CdZnTe), cadmium zinc selenium telluride (CdZnSeTe) which are II-VI compound semiconductors are attracting attention in recent years as promising materials. Although these are referred to as CdTe-based semiconductors herein, compared to semiconductors such as silicon (Si) and germanium (Ge) and the like, these semiconductors have the merits of (1) high detection efficiency of radiation since their atomic number is relatively large, (2) a cooling device is not necessary since the band gap is large and the influence of thermal noise current is small. When radiation enters in a state in which an electric field is applied to a semiconductor attached with electrodes, electrons and holes are generated in the semiconductor and mutually proceed along the applied electric field in opposite directions. In this way, radiation is detected by extracting the induced current as a signal.
In the growth process of a CdTe-based compound single crystal which is a CdTe-based semiconductor, high vapor pressure Cd easily escapes from a single crystal and inevitably Te precipitates are generated in the single crystal. In the conventional growth technique of a single crystal, efforts have been made to reduce the precipitate size by controlling Cd vapor pressure during crystal growth (U.S. Pat. No. 3,232,461).
When Te precipitates are present in a CdTe-based compound single crystal, a drop in radiation detection sensitivity occurs in the region where Te precipitates are present, and in the case when a CdTe-based compound single crystal substrate is used in a radiation detection element for imaging, defects in the image are likely to occur. The pitch size of pixel electrodes which are required for a CdTe-based compound single crystal substrate to be used for imaging sensor applications have become smaller year by year, miniaturization in the size of the Te precipitates and a reduction in a Te precipitate concentration are required.
In the conventional growth technique of a single crystal, while a technique has been reported in which a CdTe-based compound single crystal is grown while applying a Cd vapor pressure, and furthermore, after single crystal growth, Te precipitates are miniaturized by applying a Cd vapor pressure and performing a heat treatment (post annealing), a technique for miniaturizing Te precipitates or getting rid of the Te precipitates and sufficiently reducing the Te precipitate concentration has not been reached (U.S. Pat. No. 2,832,241).
Here, a substrate having a high resistance is demanded for a CdTe-based compound single crystal for use in radiation detector applications. A CdTe-based compound single crystal shows p-type conductivity electrical properties in which Cd vacancies which are one crystal defect within the single crystal immediately after the growth (as-grown) are present, which contribute to the p-type conductivity. In order to make a CdTe-based compound single crystal high resistance, by adding impurities (for example, chlorine) which contribute to n-type conductivity as a dopant, and compensating for carriers by a p-type acceptor of Cd vacancies and the n-type donor of chlorine element, it is possible to manufacture a high resistance CdTe-based compound single crystal useful for radiation detector applications. However, the problem of the precipitates described above has not been solved even for a CdTe-based compound single crystal with the addition of impurities (Japanese Laid Open Patent Publication No. 2004-238268).
When annealing is performed while applying a Cd vapor pressure in order to miniaturize Te precipitates or reduce the concentration of Te precipitates, even if it is effective in reducing Te precipitates, the vacancy concentration of Cd also changes, the compensation relationship of the carriers collapses which is likely to make n-type and realizing a high resistance is difficult.